Start blame on the commit passed to a "tig show" commandline

Fixes the first issue described in #1127 (comment)

NEWS.adoc

@@ -1,6 +1,14 @@
 Release notes
 =============
 
+master
+------
+
+Improvements:
+
+Bug fixes:
+- Start blame on the commit passed to a `tig show` commandline (#1135).
+
 tig-2.5.4
 ---------
 

src/diff.c

@@ -641,7 +641,8 @@ diff_trace_origin(struct view *view, struct line *line)
 	int chunk_marker = line->type == LINE_DIFF_DEL ? '-' : '+';
 	unsigned long lineno = 0;
 	const char *file = NULL;
-	char ref[SIZEOF_REF];
+	const char *ref = view->vid;
+	char blame_ref[SIZEOF_REF];
 	struct blame_header header;
 	struct blame_commit commit;
 
@@ -686,18 +687,21 @@ diff_trace_origin(struct view *view, struct line *line)
 		}
 	}
 
+	// HACK-ish: we cannot use is_initial_view here, because view->argv is already set.
+	if (!view->prev && opt_rev_args)
+		ref = opt_rev_args[0];
 	if (chunk_marker == '+')
-		string_copy(ref, view->vid);
+		string_copy(blame_ref, ref);
 	else
-		string_format(ref, "%s^", view->vid);
+		string_format(blame_ref, "%s^", ref);
 
-	if (string_rev_is_null(ref)) {
+	if (string_rev_is_null(blame_ref)) {
 		string_ncopy(view->env->file, file, strlen(file));
 		string_copy(view->env->ref, "");
 		view->env->goto_lineno = lineno - 1;
 
 	} else {
-		if (!diff_blame_line(ref, file, lineno, &header, &commit)) {
+		if (!diff_blame_line(blame_ref, file, lineno, &header, &commit)) {
 			report("Failed to read blame data");
 			return REQ_NONE;
 		}